BUDAPEST, Nov 20 (Reuters) - Hungary’s central bank kept its key interest rates on hold at record lows on Tuesday, but some investors said it may soon need to signal a start to monetary tightening in the face of significant upside surprises in economic growth and inflation.

Central Europe’s most dovish central bank left its base rate at 0.9 percent and its overnight deposit rate at -0.15 percent , both in line with the median forecasts of analysts in a Reuters poll.

The bank reiterated that it was prepared for a “gradual and cautious” normalisation of its monetary policy depending on the outlook for inflation. But it did not say when a change in policy would start.

“The inflation target is still expected to be achieved in a sustainable manner from mid-2019. To ensure this ... maintaining the current level of the base rate and the loose monetary conditions is necessary,” the monetary council said in a statement.

“Inflation is expected to decrease in the coming months due to the fall in fuel prices, while the measures of underlying inflation are likely to rise,” it added.

At 1424 GMT, the forint traded at 321.35 versus the euro, unchanged from levels before the rate announcement.

At 4.8 percent, Hungary’s third-quarter economic growth came in well above a 4.3-percent market forecast.

Meanwhile, annual inflation rose to 3.8 percent in October, exceeding both the market consensus and the central bank’s own monthly forecast for the second consecutive month.

The October reading also skirted the top of the bank’s 2-4 percent target range.

The bank said inflation was expected to decrease in November and December but remain slightly above 3 percent. It said inflation expectations remained anchored at low levels.

The central bank has signalled that any major changes to its policy toolkit would come at a quarterly review of inflation trends, with the next such sitting due in December.

Goldman Sachs said that in light of the recent inflation data it now expects the first rate hike by the NBH in the third quarter of 2019 rather than the fourth quarter.

“Recent inflation developments will put pressure on the NBH to bring forward its guidance for the commencement of monetary tightening at the December meeting,” Goldman Sachs said.

The consensus forecast of analysts polled by Reuters shows the NBH’s base rate is unlikely to change before 2020.

But the bank might tighten conditions before then, by reducing the liquidity it provides to banks through its currency swap facility, or raising its overnight deposit rate.